1. Field of Invention
This invention is directed to the fabrication of III-V nitride semiconductor films on mesa structures for use in visible light emitting optoelectronic devices. More particularly, this invention is directed to III-V nitride structures.
2. Description of Related Art
High-efficiency solid-state electronic devices based on III-V nitrides have been fabricated that have a sufficiently wide bandgap for short-wavelength visible light emission. III-V nitrides have been used in light-emitting diode (LED) devices to provide bright green-to-ultraviolet light at high efficiencies. Known LEDs cover all three primary colors of the spectrum (red, green and blue). Accordingly, they can be used in various combinations to produce any color. III-V nitrides have also made possible diode lasers that operate at room temperature and emit shorter-wavelength visible light in the blue-violet range under continuous operation. These diode lasers emit coherent radiation and focus light into smaller spots, enabling higher-density optical information storage and printing. Blue lasers are particularly promising in this regard due to their short wavelength. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers.
III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under high electric current and intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
GaN/AlGaN epitaxial films have been grown on sapphire substrates for optoelectreonic applications. Presently, a and c-oriented single crystal sapphire is the most commonly used substrate. Diode lasers formed using group III-V nitride films present problems because these lasers are inefficient and have a high current threshold. Accordingly, a high current is required to produce gain. This fact, coupled with the relatively high electrical resistance and the poor thermal conductivity of sapphire substrates makes heat generation a problem in these lasers. To increase laser efficiency, injected electrical carrier and optical confinement at the active regions needs to be increased, to reduce laser current threshold and related heating effects.
A known approach to increasing confinement at active regions in lasers is to increase the aluminum percentage in AlGaN cladding layers surrounding InGaN active regions. Increasing the aluminum percentage too much, however, can degrade the structure and optical properties of the epitaxial films. The epitaxial films become severely cracked for aluminum precentages of about 10% or greater.